The invention relates to a process for the work-up of isocyanate adducts such as polyurethanes, residues from isocyanate production, in particular distillation residues from the preparation of tolylene diisocyanate (TDI) or hexamethylene diisocyanate (HDI).
Isocyanate adducts are obtained in large quantities as waste material in industry. Examples are polyurethane foams, here, for example, production scrap or foams from redundant appliances, motor vehicles or furniture.
A further group of isocyanate adducts is made up of production wastes, in particular distillation residues from the preparation of polyisocyanates, in particular tolylene diisocyanate (TDI) or hexamethylene diisocyanate (HDI). Particularly in the preparation of TDI, one of the most used polyisocyanates, a large amount of residues is obtained.
TDI is used in large quantities for producing polyurethanes, in particular flexible polyurethane foams. The preparation of TDI is usually carried out by reacting tolylenediamine (TDA) with phosgene. This process has been known for a long time and is widely described in the literature.
For this purpose, the TDA is usually reacted with phosgene in a conventional two-stage phosgenation.
At the end of the synthesis, there is usually a distillation step in which TDI is separated off from high-boiling by-products. For process engineering reasons, for example to ensure the pumpability of the residue, the residue can still comprise up to 70%, preferably up to 50%, particularly preferably up to 30%, of TDI. Thus, there is a considerable economic incentive to make use of the materials in this residue in the case of present-day “world scale plants” having an annual capacity of up to several hundred thousand metric tons.
One possibility which is frequently employed to recover at least part of the TDI comprised in the distillation residue is to remove more of the TDI from the residue, for example by means of an extruder. Suitable apparatuses are, for example, List dryers. These are specific paddle dryers from List which are frequently used in isocyanate production. The amount of TDI in the distillation residue can be significantly reduced in this way. However, this process also produces a generally solid residue, as a result of which the yield of the process is reduced. This residue has hitherto usually been burnt.
An alternative way of utilizing the distillation residues is to make use of the materials there. Various processes are known for this.
One such possibility for utilization is reaction of the residue with water, known as hydrolysis. Such processes are widely described. Hydrolysis of the residue is promoted by bases or acids. Amines also promote hydrolysis. Hydrolysis can be used for denaturing the TDI distillation residue, as described, for example, in U.S. Pat. No. 4,091,009. A further possibility is the recovery of TDA which can then be reacted again with phosgene to form TDI. Such processes are described, for example, in DE-A 29 42 678, JP-A 5 8201 751 and DE-A 19 62 598.
DE-A 27 03 313 describes a hydrolysis process which can be carried out either batchwise in an autoclave or continuously in a tube reactor. The hydrolysis of the solid TDI residue is carried out using aqueous ammonia solution, solutions of primary or secondary amines in water or aqueous TDA solution.
U.S. Pat. No. 4,654,443 describes a hydrolysis process in which the TDI residue is reacted with TDA to form a solid in a first process step and this intermediate is hydrolyzed with water in a second step.
The hydrolysis enables a large part of the products of value comprised in the residue to be recovered. However, complete transformation is not possible in the known processes, so that products of value are still lost.
WO 2006/134137 describes a process for the work-up of isocyanate residues, in which monomeric isocyanate is firstly separated off from the residue in a List dryer and the residue obtained is treated with ammonia. In this process, too, there is only incomplete recovery of the materials of value, and in addition ammonia is difficult to handle.
It was an object of the present invention to develop a process for the work-up of residues from the production of isocyanates, in which a higher yield can be achieved and which is easy to manage.